Morphology, photoluminescence and electronic structure in oxidized siliconnanoclusters

The dependence of quantum size effects on bonding structure in oxidized sil icon nanoclusters is established by correlating photoluminescence data with photon-yield electronic structure measurements at the advanced light sourc e. The nanoclusters were synthesized using a laser ablation technique that utilizes a convective He environment to control the size of the particles. After removal from the growth chamber, our ex situ photoluminescence (PL) r esults indicate that, as the nanoclusters oxidize, the main PL peak moves f rom 1.83 to 1.94 eV in energy. The central focus of the present work is to establish the origin of the main PL peak, and to determine why its energy s hifts as the nanoclusters are allowed to oxidize slowly in air. The changes in the morphology and bonding structure of the clusters was established us ing soft-X-ray fluorescence spectroscopy (SXF) and photon-yield near-edge X -ray absorption fine structure (NEXAFS) spectroscopy, which probe the eleme nt-specific density of occupied (SXF) and unoccupied (NEXAFS) electronic st ructure. Our conclusion is that the as-synthesized nanoclusters consist of a pure, crystalline Si core within a nearly pure SiO2 shell, with little or no sub-oxides present. As the nanoclusters oxidize, the radius of the crys talline core decreases in size, which gives rise to the change in the posit ion of the PL signal. (C) 2001 Elsevier Science B.V. All rights reserved.